Latex-based paints have captured a significant portion of both the indoor and outdoor paint markets due to a number of significant advantages these paints possess over the solvent-based paint products. The primary advantages are low odor, easy clean-up, and fast dry, however, the paints suffer from the drawback of poor "wet adhesion".
Latex-based paints generally comprise three components, an emulsion polymer or copolymer which is film forming (the latex), an aqueous phase (which contains, inter alia, additives such as pigments, defoamers and antifreeze agents) and one or more surfactants. Various additives are often contained in the formation as particular applications dictate.
Two types of emulsion polymers are generally used in formulating latex paints. The first type comprises copolymers of alkyl esters of acrylic and methacrylic acid with minor amounts of acrylic and methacrylic acid and are termed "all acrylic". All acrylic paints produce hard coatings which are resistant to scrubbing and provide good water resistance and wet adhesion. However, because of the cost of the acrylic monomers all acrylic paints are mainly used in premium or high quality paints.
The second type of latex comprises vinyl acetate terpolymers in combination with softer acrylic monomers. This type of latex, often called "vinyl acrylic", are cheaper than the all acrylics. The paints produce films which are hard, but tend to have a much lower wet adhesion than their acrylic based counterparts.
The term "wet adhesion" is used in the paint industry to describe the ability of a paint to retain its adhesive bond (to the substrate) under wet (or high humidity) conditions. Organic solvent-based paints generally exhibit good wet-adhesion but water-based (latex) paints often tend to exhibit reduced wet-adhesion under wet or humid conditions. This inherent deficiency severly limits the utility of latex paints in areas where humid environments might be encountered, e.g. exteriors, bathrooms and kitchens. Further, painted surfaces which become soiled must be washed and cleaned, often using water and abrasive scrubbing compounds. Because of their tendency to have lower wet-adhesion, the latex paints are also much less scrub-resistant than their organic solvent based counterparts.
Much effort has been devoted in recent years to improving the wet adhesion of latex-based paints. Some researchers have attempted to vary various paint formulation component parameters, such as pigment types, dispersant types, and coalescing agents to improve wet adhesion; however the most significant improvements in wet adhesion properties has been obtained by the functional modification of the copolymeric backbone of the latex. Specifically, incorporation of amine, amide, and acetoacetate functionalities into the copolymer have shown promising results. For example, cyclic ureido derivatives have been described in the patent literature as imparting wet adhesion properties, e.g. U.S. Pat. Nos. 4,104,220, 4,111,877, 4,219,454, 4,319,032 and 4,599,417.
These compounds are described as improving wet adhesion properties for vinyl acetate terpolymers when copolymerized with them. However, wet adhesion monomers are very expensive and their inclusion results in a drastic increase in the cost of the vinyl acetate terpolymers and all-acrylic copolymers used in the paint.
A further problem observed with latex paints is that of destabilization due to freezing and thawing. During freeze-thaw cycling, the anionic surfactants commonly employed in latex paints tend to desorp from the latex particles, thereby increasing the ionic strength of the aqueous phase, ultimately resulting in paint coagulation due to destabilization of the electronic double layer surrounding each particle. Thus, the viscosity of the paint will vary as more freeze/thaw cycles are encountered. Ordinarily, antifreeze compounds (e.g. ethylene glycol) are added to the paints to combat this, but this contributes to the cost of the paint and also requires the use of volatile organic compounds.
Researchers have attempted to alleviate this problem by using blends of surfactants in place of the anionic surfactant. For example, U.S. Pat. No. 3,193,517 discloses a method for making a viscosity-stable latex by blending a cationic surfactant, an amphoteric surfactant, and a nonionic surfactant in a prescribed ratio. However, the blended system is not entirely satisfactory in many other respects.
There exists a real need for both wet-adhesion promoters and viscosity stabilizers for use in latex paints.
The onset of paint coagulation can therefore be delayed or eliminated entirely.
The amphoteric surfactants of this invention can thus be used to produce latex paints having wet adhesion values in the range of the alkyds, even when vinyl acrylics are used in the paint, which also are more stable toward freeze/thaw cycling than conventional latex paints.